In paving or repaving surfaces with asphaltic concrete or other thermoplastic materials it is a common practice to transport paving constituents to a fixed preparation plant and then hot mix from the plant is trucked to the paving site. Transport costs for such purposes are substantially greater than would be the case if the materials were hauled directly to the paving site and then heated and mixed at the paving site in an economically efficient manner.
Disadvantages of this conventional paving procedure are not limited to the typically high hauling costs. As the hot mix tends to cool while being trucked to the paving site, it is often heated to a higher temperature at the plant than is required during paving operations at the distant work site. Alternately, elaborate hot mix transporting vehicles may be used which continue to heat the material en route to the paving site and which also continue to mix the material since cooling tends to be uneven. Because of cooling which occurs en route to the paving site, different batches of material may arrive at somewhat different temperatures and there may be undesirable temperature differentials within a single batch of materials. This means that process control of temperatures at the paving site is far from ideal and this may adversely affect the quality of the pavement.
It has heretofore been proposed to truck pavement constitutents directly to the work site at which the pavement is to be laid down and to heat and mix the materials at that location using combustion heaters. This has not become a widespread practice except for the patching of very small localized areas of a roadway or the like. It is believed that such a practice has not been widely adopted at least in part because of a lack of economically feasible methods or apparatus for quickly and efficiently heating paving materials at the actual paving site. Some of the transportation costs may be reduced and some of the other adverse factors discussed above may be alleviated by paving with cold mixes, but these often do not provide as high a quality pavement as hot mixes and also tend to constrict flexibility of process control at the paving operation. For example, paving operations with any given batch of cold mix must be accomplished within restrictive time constraints before the emulsification process has proceeded to a critical point.
My copending U.S. patent application Ser. No. 756,365, filed Jan. 3, 1977 and entitled MICROWAVE METHOD AND APPARATUS FOR REPROCESSING PAVEMENTS, discloses methods and apparatus for repaving operations at deteriorated roadways or the like which greatly reduce or even eliminate the cost of transporting paving materials by recycling the old pavement at the site. In the method of my above-identified copending application, deteriorated asphaltic concrete pavement is decomposed in a very rapid and efficient manner by directing microwave energy downwardly into the old pavement. The old pavement constituents are then remixed in place on the roadbed or the like and recompacted to provide a high quality repaved surface.
It has not been recognized in the industry that work site heating of paving materials with microwave energy can also provide very substantial cost reduction in connection with the initial paving of surfaces with new materials or in situations where a layer of additional paving materials is to be applied over old pavement. Apparatus disclosed in my above-identified copending application may be used for this purpose. New or reclaimed paving materials in an unheated condition may be spread out in a layer of the desired thickness on the surface which is to be paved. The apparatus of my copending application may then be traveled along the surface to be paved while heating the layer of material very rapidly and uniformly by directing microwave energy downwardly into the material. The heated materials may then be mixed on the roadbed and then compacted to form the layer of new pavement. Substantial cost economies may be realized by this technique. Costs may be further reduced and energy efficiency may be further enhanced by recovering heat from the exhaust of the engines which drive the generators to power the microwave sources and then using such heat to supplement the microwave heating of the paving materials.
It is desirable to reduce paving costs still further, particularly by further increasing energy efficiency in view of the growing scarcity of energy resources. I have now recognized that a significant unproductive dissipation of energy may occur in the on site heating of paving materials with microwave energy as described above, the reduction of which would still further increase energy efficiency and reduce costs. In particular, I have observed that the desired thickness of a layer of new paving to be applied to a surface is often less than the depth to which the microwave energy penetrates downwardly through the material and into the underlying surface. While some heating of the extreme upper portions of the underlying surface may be desirable to assure good bonding with the new paving, the microwave heating of the deeper portions of the underlying surface is unnecessary and results in an unproductive dissipation of costly microwave energy.
Aside from energy considerations, on site microwave heating of paving materials according to the above-described technique is subject to a further complication in specialized circumstances where a wall, bridge framing or other obstruction adjoins the area to be paved. As broad trapping structures are present at the sides of the microwave energy applicator to prevent an outward broadcasting of microwave energy, the edges of the area are not directly heated by microwave energy.